[FLINK-25054][table-planner] Extend the exception message for SHA2 function

[xishuaidelin]

What is the purpose of the change

This pull request extends the exception message for sha2 function.

Brief change log

modify the function addReusableSha2MessageDigest.

Verifying this change

N/A

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): ( no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): ( no)
• The serializers: (no )
• The runtime per-record code paths (performance sensitive): (no )
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (no )
• The S3 file system connector: (no )

Documentation

• Does this pull request introduce a new feature? (no)
• If yes, how is the feature documented? (not documented)

[flinkbot]

CI report:

• 871683417cee8f6d106e786a32c6a3035d3675f5 Azure: SUCCESS

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